Deep photometric surveys in the submm and FIR have identified a previously unknown population of distant, dusty galaxies that emit the bulk of their significant bolometric luminosity in the rest-frame IR. Determining the nature of these sources along with what powers their prodigious
submm/FIR output is key to securing the place of this cosmologically significant population on the evolutionary pathway of galaxies. Deep MIR and FIR spectroscopy presents the best prospects for interrogating this distant and highly obscured population. In this paper we present he preliminary results of a quantitative study to determine how efficient SAFARI will be at making deep spectroscopic surveys in the MIR and FIR, and introduce a new SAFARI-specific diagnostic from the rest-frame MIR that can be used to differentiate between AGN and starbursts using lines that fall in the SAFARI waveband in objects at z > 0.4.